Growing environmental concern makes it more and more difficult to dispose of hazardous waste in direct land-filling, by storage in surface impoundments, by deep well injection, by dumping or simple burning off at sea.
It is therefore expected that in the future, incineration or thermal oxidation at high temperature will play an increasing role as an environmentally acceptable way of destroying hazardous organic waste.
Incineration is an engineered process that employs thermal decomposition via thermal oxidation at high temperature (normally 900.degree. C./1650.degree. F. or greater) to destroy the organic fraction of waste and reduce volume. Generally, combustible wastes or wastes with high organic content are considered most appropriate for incineration. However, technically speaking, any waste with a hazardous organic fraction, no matter how small, is at least a functional candidate for incineration. For instance, significant amounts of contaminated water are currently incinerated in the United States. Contaminated soils are also being incinerated with increasing frequency.
The minimum temperature required for incineration ranges from 875.degree. C./1610.degree. F. for incineration of municipal garbage, to 1400.degree. C./2550.degree. F. for the incineration of the most stable organic components such as PCB, dioxin, DDT and residues from the production of polyvinyl halogenides. The residence time at the high temperature should be at least 2 seconds.
A separate technology, that of producing cement clinker in cement kilns, also involves high temperature burning. While liquid wastes have in the past been burned in cement kilns, no effort has been made to employ cement production in conjunction with the destruction of highly hazardous solid or semi-solid contaminants. The reason is that while liquid waste can be fired using conventional oil burners similar to fuel oil burners, no technology for introducing solid or semi-solid hazardous waste into the burning zone of rotary kilns has been available. The scope of the present invention is to provide such a technology.
In this regard, the incinerating of liquid contaminants in cement kilns has been as much for the purpose of obtaining inexpensive fuel for the kiln. Thus a cement kiln operator may be willing to actually pay for contaminated oil or the like.
The burning of liquid organic hazardous wastes allows for the recovery of substantial amounts of energy from waste materials. The production of cement is a highly energy intensive process. Energy costs generally account for 33 to 40 percent of the cost of producing clinker. Waste fuels typically have a heat content comparable to coal. By injecting liquid organic wastes into the kiln as a supplemental fuel, cement kilns may greatly reduce their use of primary fuels, thus providing substantial savings to the manufacturer.
The present status of hazardous waste incineration in cement kilns is that only liquid waste is burned in the firing end of the kiln. Most of the kilns burning liquid waste are wet process kilns, at least in the United States. The reason is presumably that wet process kilns are particularly fuel inefficient, for which reason operators of wet kilns have been strongly motivated to find ways to lower their fuel bill. Another reason is that wet kilns can accept high inputs of chloride without expensive modifications (up to 0.8% Cl on clinker basis), whereas dry process kilns with 4-stage preheaters are limited to a chloride input of 0.015% (precalciner kilns) to 0.023% (preheater kilns) on clinker basis. If the chloride input exceeds these figures, the kilns will have to be equipped with costly by-pass installations which in the most extreme case (a precalciner kiln with 100% by-pass for kiln tube gas) will permit a total chloride input of about 0.8% Cl on clinker basis.
The gas exit temperature from the kiln tube typically is about 1100.degree. C., which however drops to 820.degree. C. (preheater kilns) or 900.degree. C. (precalciner kilns) a few tenths of a second after the gas leaves the kiln tube (see FIG. 1) when raw meal is introduced into the stream of kiln gas. The gas taken out through the by-pass is quenched with cold air to even lower temperatures to permit the handling of the by-pass gas.
For this reason it is not possible to introduce chlorinated waste into the back end of preheater kilns, or to fire it into a precalciner since a temperature of at least 1200.degree. C. at 2 seconds gas retention time is required. Non-chlorinated solvents (as for instance turpentine, xylenole, etc.) may be burned in precalciners or introduced into the back end of preheater kilns without problems, and non-chlorinated solid waste may be used in the same way. It has for some years been common to introduce automobile tires in the back end of preheater kilns where they burn without problems.
Cement kilns have the potential to dispose of large volumes of organic waste. For example, by replacing 35 percent of its coal consumption by burning waste fuels, a typical cement kiln could burn on the order of 100 tons of hazardous wastes per day. Such substitution rates have been achieved with liquid organic waste.
The situation changes drastically, however, when solid or semi-solid residues are concerned. With respect to these, companies may be required to pay ransom amounts to have them removed and destroyed in an environmentally acceptable manor. The lack of a heretofore available effective and low cost means of disposing of such wastes has resulted not only in high removal and treatment costs, but in unsafe or illegal dumping with almost incalculable damage to the environment.